One method of measuring an unknown force comprises applying the force to some elastic member and measuring the resultant deflection or strain. Conventional strain gages are used to measure the effects of unknown forces on devices and structures. Strain gages can be used either to determine the strain caused to a device by loads during its operation or they can be used to determine an unknown force in a wind tunnel by measuring how the force affects a force balance fixture. Similar force balance fixtures are used in other applications.
A common problem with force balances is that they are subject to overload grounding and damage as well as some other limitations. Under some conditions momentary high loads cause fixture grounding which damages the strain gages and disrupts force balance calibration.
It is an object of the present invention, therefore, to Provide a force balance that is substantially uneffected by momentary overloads and groundings.
Conventional wire strain gages have been very effective in measuring strains in most applications. Conventional strain gages are of limited use, however, when they are applied to very stiff structures. The normal sensitivity range of conventional strain gages is from about 2.times.10.sup.-3 to 10.sup.-6 inches per inch. The upper limit of 2.times.10.sup.-3 inches per inch, approaches the elastic limit for high strength steel used in force balance fixtures. At the lower limit, strain gage bridge output drops into the region where the output is made unreliable due to Seebeck potentials and zero drifts related to imperfect matching of resistance, gage factors and temperature effects. This limits strain gage utility for very stiff structures and for measuring very small forces. Small forces which, for example, may be almost insignificant in wind tunnel testing can be very significant in flight. A need therefore exists for very sensitive force detection beyond that which is currently possible with conventional strain gages.
Large non-elastic structures such as concrete buildings are sometimes unable to bear very much strain prior to failure. Minute amounts of strain may therefore be significant in determining the use and predicted life expectancy of non-eleastic structures. A need therefore exists for a means of measuring very small amounts of strain.
In view of the above it is an object of this invention to provide a means particularly suitable for measuring very small forces and minute amounts of strain.
It is a further object of this invention to provide a means for determining strain in very stiff structures.